Various chemical substances have heretofore been employed as hardening agents for epoxy resins. Most usually these hardening agents have been organic acid anhydrides or amines, although other agents have been used. While some of the hardening agents have been described as being water dispersible (U.S. Pat. No. 3,467,684), such hardening agents have a number of drawbacks when mixed and used with water dispersed or emulsified epoxy resins. The viscosity of the resultant mix increases rapidly and gelation of the mixed components occurs in a very short time, because of the lack of compatability between the two coreactants. As the mutual solubility of the two coreactants decreases, the viscosity of the mixture increases greatly, which in turn reduces or diminishes the desirable film characteristics of the mixture. While this problem can be overcome in part through the use of solvents, the inclusion of even small amounts of organic solvents, as is required by the presently available curing agents, creates an odor and fume problem, which is particularly troublesome when the compositions are used in such odor-sensitive areas as hospitals, bakeries, kitchens, and the like.
In addition to the fact that the prior art compositions have a short pot life, the gloss of coatings produced therefrom varies during the life of the mixed batch. The coatings resulting from coating compositions produced from the initial mixing of the prior art curing agents and the resin emulsion or dispersion have a high gloss, but as the work progresses during the application, the gloss of the coating decreases and the coatings produced from compositions near the end of their pot life have low gloss. This problem is especially noticeable when large areas are being coated and is further manifested when touch-up work is undertaken, wherein the touch-up paint will not match the original paint with respect to the gloss.
The novel curing agents of the present invention have greatly increased solubility or dispersibility in water, which allows the reaction between the new curing agent and the epoxy water emulsion (or dispersion) to proceed at a controlled rate. Therefore, use of these novel curing agents will result in a substantial increase in the usable room temperature pot life of the mixed system. The coating systems formed using the novel curing agents of the present invention cure readily at ambient conditions to produce film properties such as gloss, adhesion, chemical resistance, scrubability, and flexibility which are greatly enhanced; in fact, some of the cured film properties of epoxy coating systems utilizing these novel curing agents will actually exceed those of comparable solvent-based epoxy coating systems. In addition, the coating system cured with the curing agents of the present invention will produce uniformly good film properties whether applied soon after mixing the coreactants or applied near the end of the usable pot life of the mixture.